In all mass spectrometers, an ion source is used to generate an ion beam characteristic of the composition of the sample, and this ion beam is transmitted to an ion detector via a mass filter placed between the source and the detector. The mass filter may be one of several different types. A commonly employed type is based on a magnetic sector analyser, which selects ions on the basis of their momentum. The velocity of the ions passing through the sector must therefore be maintained at a constant value in order that the resolution is not degraded, and an electric sector analyser, which allows the passage only of ions having a particular kinetic energy, is often used in conjunction with a magnetic sector analyser for this purpose. In contrast, a quadrupole mass filter separates ions on the basis of their mass to charge ratios only, and involves the passage of the ions through an alternating electric field at radio frequency (RF). For certain applications, mass spectrometers based on this principle have a number of advantages over other types, especially where very high mass resolution is not required, and where fast scans of a range of masses is needed. A quadrupole mass filter consists of four electrically conductive electrode rods arranged symmetrically about, and very accurately parallel to the line joining the ion source to the detector. Opposite pairs of the rods are electrically connected together, and an electrical potential oscillating at radio frequency, together with a superimposed direct voltage, is applied between them. Designating three axes of rectangular coordinates x, y, and z, so that the x axis is the line joining the centres of the rods which are connected to the positive pole of the direct voltage supply, the y axis is the line joining the centres of the rods which are connected to the negative pole of the direct voltage supply, and the z axis is the line joining the ion source to the detector, then a positively charged ion entering the analyser along the z axis with a finite velocity will be subject to a combination of electrical forces and will describe a complex trajectory. This trajectory is best considered as the resultant of two motions, one in the y-z plane, and the other in the x-z plane. Assuming first that only the direct voltage is applied, then the motion of the ion in the x-z plane, where the rods are positively charged, will be simple harmonic in character, and the trajectory will be stable, that is, remaining finite in amplitude. However, the motion of the positive ion in the y-z plane, where the rods are negatively charged, will be divergent away from the z axis, with constantly increasing deviation, so that the trajectory is unstable and the ion will be lost by striking one of the rods. If, on the other hand, only the RF field is applied, then the trajectories in both planes will be alternately deflected towards and away from the z axis, and a stable trajectory in both planes is possible providing the frequency is high enough and the ion is heavy enough not to respond sufficiently during the defocussing part of the cycle to strike one of the rods. If both the direct voltage and the RF voltages are simultaneously applied, then the potential between the pairs of rods at any instant will be given by EQU .phi.=U+Vcos wt,
in which:
U is the direct voltage; PA0 V is the zero-to-peak RF voltage; PA0 and w is the angular frequency of the applied RF (=2.pi.f, where f is the frequency in Hz.) PA0 m is the mass of the ion; PA0 r.sub.o is the radius of the field (i.e., one half the distance between the inside surfaces of the rods); and PA0 U,V, and w are as defined previously.
The light ions will be able to follow the alternating component. In the x-z plane they will tend to have unstable trajectories whenever the alternating component exceeds the direct component, and eventually strike the rods, so that only heavy ions will pass through the filter without being lost by striking the x electrodes. However, in the y-z plane, the trajectory of heavy ions tends to be unstable because of the defocussing effect of the direct component, but some of the lighter ion trajectories will be stable because they will be corrected by the RF component whenever their amplitude tends to increase. Thus the quadrupole filter acts as a combination of a high pass and a low pass mass filter, and will only transmit ions of a certain range of mass to charge (m/e) ratios. The behaviour of the filter can be treated theoretically, for example, as described by Paul et. al. in U.S. Pat. No. 2,939,952, and a drawing indicating the operating conditions where stable trajectories exist can be constructed. Such a drawing is shown in FIG. 1, which is a plot of a parameter a against parameter q, which are given by the expressions: ##EQU1## in which: e is the charge on the electron;
Clearly, for ions to be transmitted through the filter, the ion trajectories in the x-z and y-z planes must be simultaneously stable, and for a given geometrical arrangement and frequency (i.e., r.sub.o and w constant), it is clear from FIG. 1 that transmission can occur in any of the regions where x stability and y stability regions overlap. However, in practice, only the cross shaded region close to the origin is used because of practical limitations.
The parameters a and q plotted in FIG. 1 are both inversely proportional to the m/e of the ion, and an alternative method of indicating the stable region is on a plot of U, the direct voltage, against V, the RF potential, for ions of particular m/e values, at constant r.sub.o and w. FIG. 2 shows two such plots, limited to the stable region close to the origin of FIG. 1, for ions of m/e=2 and m/e=28. It is clear from the figure that the maximum resolution, which corresponds to the minimum range of m/e values transmitted, is obtained by increasing the ratio of U/V to the point A. At this point the trajectories of ions only slightly heavier than the one transmitted become unstable in the y-z plane, and the trajectories of ions only slightly lighter then that transmitted become unstable in the x-z plane. From the above equations, it can be seen that EQU a/q=2U/V
so that the point of maximum resolution is independent of the mass of the ion transmitted. The resolution can be lowered by reducing the value of U/V. FIG. 2 shows that the m/e ratio transmitted is dependent on V, but that the point of maximum resolution always occurs at the same ratio of U/V, as indicated by the dotted line. The quadrupole may therefore be scanned by varying V, but keeping the ratio U/V constant at a value which maintains the desired resolution. Alternatively, the U and V values may be scanned along a line parallel to the dotted line in FIG. 2, but displaced downwards slightly so that it cuts the V axis between points O and B. This mode of scanning results in peaks of a certain constant width, and is commonly used to obtain unit mass resolution over the entire mass range of the filter. It is the conventional mode of operating a quadrupole mass analyser.
In certain applications, however, including using a small quadrupole instrument for residual gas analysis, it is also useful to operate the quadrupole in the RF only mode, that is with U=0. In this mode, it acts as a broadband high pass mass filter, which passes all masses above a certain mass value. For example, referring to FIG. 2, an ion of m/e=28 will be transmitted if the RF voltage lies anywhere between points O and C, but an ion of m/e=2 will only be transmitted if the RF voltage is less than B. Operation of the quadrupole with an RF voltage below B, and U=0 should therefore result in the transmission of all ions above m/e=2, and the signal reaching the detector will be the sum of the intensities of all the ionized species produced by the source. In the case of a quadrupole used as a residual gas analyser in an ultra high vacuum system, this mode of operation can be used to produce a measure of the total pressure in the system, eliminating the need for a separate pressure gauge, such as an ion gauge. Other uses for quadrupoles used in the RF only mode include high efficiency transmission devices used to transmit all ions of a particular range of m/e values, for example in mass spectrometers used for the study of ion-molecule reactions, etc., such as that described in U.S. Pat. No. 4,234,791.
In practice, however, the behaviour of a real quadrupole analyser operated in this mode departs from the ideal. FIG. 3, which is a plot of the detected signal intensity against the RF potential V for ions of m/e=2 and m/e=28 suggests that in the ideal case, each ion is transmitted with the same efficiency over the range of RF potential values from 0 to the appropriate limiting value. However, curves for samples of hydrogen, helium, nitrogen, air, and carbon dioxide obtained in practice with a small quadrupole of the residual gas analysis type, shown in FIG. 4, differ considerably from the ideal shape shown in FIG. 3. The differences are probably caused by departures from ideal of the construction of the real quadrupole, for example, the use, to simplify manufacture, of rods of circular cross section in place of the hyperbolic rods required by the theory, and the use of simple ion sources which produce imperfectly collimated beams of ions with relatively large energy spreads. Defects in manufacture, e.g., imperfect rod alignment, may also contribute. It will be seen that the most important difference between the practical and theoretical curves is that the real quadrupole does not effectively pass ions at low values of the RF potential, and the cut-off occurs at a higher value for high mass ions such as nitrogen and carbon dioxide than for the low mass species such as hydrogen and helium. This is presumably because the focussing action of the RF field at low RF voltages is insufficient to overcome the defocussing of the beam due to the defects described. It is apparent from FIG. 4 that it is impossible to select a RF voltage which will effectively transmit both low and high mass ions simultaneously, because the value required to overcome the defocussing of the high mass ions is greater than the cut-off value for the low mass ions. However, if the RF voltage is set at the point TP in FIG. 4, the only ions not effectively transmitted will be hydrogen and helium, and in many cases, this will not be of importance. However, in other cases, for example the use of the filter as a residual gas analyser in an Ultra High Vacuum system, a serious error could be introduced in the total pressure reading because the residual gases at low pressures often contain a large proportion of hydrogen, and also helium when it is being used for leak checking.
It is the object of the present invention to provide a simple and economical method of overcoming this difficulty which allows the use of a simple quadrupole filter in the RF only mode to produce a signal which is proportional to the total ion current generated by the source, irrespective of the composition of the sample, thereby eliminating the need for an additional total pressure gauge such as an ion gauge.